The present invention relates to magneto-optical recording and reproducing method, magneto-optical memory apparatus and magneto-optical recording medium therefor, and more particularly to magneto-optical recording and reproducing method and magneto-optical memory apparatus for erasing and recording information by a magnetic field modulation method by using a laser beam and an external magnetic field and reproducing the information by using a laser beam to permit overwriting, and magneto-optical recording medium therefor.
Prior art overwritable magneto-optical disk devices include one which uses two light spots as described in IEEE Transaction on Magnetics MAG-20, Vol. 5 p 1013 (1984), and magnetic field modulation recording in which a magnetic field applied to a recording film is modulated with recording information as disclosed in JP-A-51-107121, JP-A-59-215008 and JP-B-60-48806. The present invention particularly relates to the overwritable magneto-optical recording and reproducing method and magneto-optical memory apparatus by the latter magnetic field modulation recording.
The magneto-optical recording includes a light modulation recording system in which a light intensity is changed in which a light intensity is changed in accordance with information to be recorded while an external magnetic field is continuously applied in order to record the information, and a magnetic field modulation recording system in which a polarity of the external magnetic field is changed in accordance with information to be recorded while a laser beam is continuously irradiated in order to record the information. In high density recording (in which a magnetic domain length is equal to or less than 0.5 .mu.m), a variation of the magnetic domain length of a circular magnetic domain due to the change of light power is large in the light modulation recording system and stable recording is hard to attain. On the other hand, in the magnetic field modulation recording system, the variation of the magnetic domain length of a crescent-shaped magnetic domain is much smaller than that in the light modulation recording system and stable high density recording is attained.
However, in the magnetic field modulation recording system, since the recording and reproducing are effected by a laser beam having a wavelength of 780-830 nm, the recording may be done but the reproduction of the magnetic domains recorded at a high density, with a high S/N ratio is difficult to attain. The S/N ratio decreases because the higher the rotation speed of the disk is, the large is the end portions of the crescent-shaped magnetic domain.
JP-B-62-59375 discloses a magneto-optical recording and reproducing apparatus for recording information by the light modulation recording system, in which a wavelength of a recording laser beam is longer than a wavelength of a reproducing laser beam, at least an objective lens is shared by a recording optical system and a reproducing optical system, and a beam spot size of the recording system is larger than that of the reproducing system.